This invention relates to a regulated power supply providing a relatively constant supply voltage to a load impedance, and more particularly to method and apparatus providing overload protection for a regulated power supply.
A power supply for converting an input line voltage to a relatively constant supply voltage of a different value that is applied to a load impedance generally comprises a DC-to-DC converter that is powered by input line current from an external voltage source, a pulse width modulator producing voltage pulses that drive the converter, and a voltage comparator. As the load impedance decreases, the supply voltage also decreases, as does the effective input impedance of the power supply. In order to maintain the supply voltage relatively constant under such load conditions, the comparator monitors the supply voltage for producing an error voltage that biases the modulator for increasing the width of voltage pulses, and thus the time interval that the converter draws line current. This operation causes the magnitude of line current drawn by the converter to increase. As the load impedance decreases from a light load (a high load impedance), the converter draws increasing values of line current and input power from the external source until the input impedance of the power supply is equal to the output impedance of the source. At this point, the source delivers maximum power to the power supply. Although a further decrease in load impedance causes the power supply to demand additional input power, the external source delivers additional input line current and less input power to the power supply. In order to prevent an overload condition (i.e., a low value of load impedance) locking-up the operation of the power supply such that it will not restore itself to normal regulation until it is entirely unloaded, the maximum value of line current must be limited. It is also desirable that the efficiency of the power supply be high so that maximum output power is delivered to the load. This is particularly important in a carrier subscriber telephone system where one power supply in a subscriber terminal services a plurality of subscriber station units (e.g., six) and associated handsets clustered at a common location. Since all of the six handsets are seldom off-hook at the same time in such an application, the load impedance under normal operating conditions typically varies from a high impedance such as 88 ohms when all handsets are on-hook to a low impedance such as 44 ohms when all six handsets are off-hook. A prior art current limiter senses the line current itself with a series resistor for limiting the maximum value thereof that is drawn by the power supply. This prior art technique wastes power in the series resistor which might better be converted to useful power that is delivered to the load.
An object of this invention is the provision of method and apparatus for indirectly sensing the magnitude of input current drawn by a power supply for limiting the maximum value thereof.